Atlas V Prepares to Loft NASA’s TDRS-K Satellite into Orbit (Part 2)

By Ben Evans, on January 27th, 2013

Not for nothing is TDRS-K’s objective described as “Fleet Replenishment.” This satellite is the first of its kind to be launched in more than a decade and will support voice and data communications traffic between the ground and low-Earth orbit until the middle of the next decade. Image Credit: AmericaSpace/Nathan Moeller

When NASA’s eleventh Tracking and Data Relay Satellite—known as “TDRS-K”—rises from Space Launch Complex (SLC)-41 at Cape Canaveral Air Force Station atop an Atlas V booster on 30 January, it will become the tenth of its kind to enter space and continue a proud heritage of providing near-continuous tracking, voice, and data communications relay services between ground stations and more than 20 separate users, simultaneously. These “users” include the International Space Station and NASA’s scientific showpiece, the Hubble Space Telescope. Yet, TDRS arose in the post-Apollo era at one of the most uncertain times for the U.S. human space program, and even its long-awaited maiden launch in April 1983 hung for a time under the shadow of abject failure.The concept was born in the early 1970s as one of the recommendations of a study group led by Don Hearth, then deputy director of NASA’s Langley Research Center, which charted possible post-Apollo roadmaps for America’s future in space. The group, which included astronaut Joe Allen on its panel, felt that a system of tracking and relay satellites placed into 22,000-mile geosynchronous orbits by the shuttle and operated from a single ground terminal at White Sands, N.M., would provide near-continuous voice and data traffic and eliminate the older generation of ships and costly ground stations.

The insect-like appearance of TDRS-K—here seen during final assembly and testing—is exaggerated by its cupped antennas, solar arrays, and other appendages. Photo Credit: Boeing

In fact, as well as supporting low-orbiting missions, it could relay data from satellites up to 3,000 miles above the Earth’s surface. Since the dawn of human space flight, astronauts had been out of contact with Mission Control for up to 80 percent of every orbit; furthermore, satellites had to tape record data and transmit it when they came within range of a tracking ship or ground station. As the shuttle effort gained momentum in the mid-1970s, it was envisaged that two TDRS relays—one over the equator, just off the north-eastern coast of Brazil, near the city of Fortaleza, and a second over the central Pacific Ocean, near the Phoenix Islands—would provide astronauts with space-to-ground voice and data links for between 85–98 percent of each orbit.

Yet, TDRS was no miracle worker. It could not process or adjust communications traffic in either direction. Rather, it operated as a “bent pipe” repeater, relaying signals and data between its Earth-circling users and the highly automated ground terminal. Signals processing, therefore, occurred on the ground, and the satellite’s sophistication was devoted to its very high throughput. Located in the inhospitable New Mexico desert, White Sands provided a clear line of sight with both satellites, and its limited amount of annual rainfall meant that weather conditions would not interfere with uplink or downlink.

TDRS-F—a member of the first generation of satellites—departs the shuttle’s payload bay in January 1993. This particular satellite remains operational and is due to be retired by the end of 2015. Photo Credit: NASA

It was envisaged that a pair of TDRSes—one stationed over the equator, just off the north-eastern corner of Brazil, known as “TDRS-East,” and a second over the central Pacific Ocean, near the Phoenix Islands, known as “TDRS-West”—would fill this urgent communications and tracking need. TDRS-A was launched in April 1983, but was almost lost when its Boeing-built Inertial Upper Stage (IUS) booster failed to insert it into its proper orbit. Only by using the satellite’s own hydrazine thrusters were controllers able to gradually manoeuvre it into its final “East” location, although the result was that its operational lifetime was shortened. Ongoing problems with the IUS meant that it was almost three years before the “West” satellite, TDRS-B, could be launched … and that was the primary payload aboard the ill-fated Challenger on 28 January 1986.

Two more TDRS satellites (C and D) were launched in September 1988 and March 1989, the former replacing the doddery TDRS-A in the west (slightly south of Hawaii) and the latter taking up position in the east, near Brazil. Unfortunately, TDRS-C also succumbed to anomalies which affected its Ku-band relay capability. A fourth satellite, TDRS-E, was launched in August 1991 and positioned at 175 degrees West longitude to become the primary provider of communications services over the Pacific from October 1991. TDRS-A and TDRS-C, meanwhile, were relegated to the status of on-orbit “spares.”

This left only TDRS-D and TDRS-E in fully-operational status … and this meant that no spare existed to support them in the event of problems. The successful arrivals of TDRS-F in January 1993 and TDRS-G in July 1995 filled this backup capability. This enabled the network to be rearranged to include two fully-operational satellites in the East and West spots, plus the fully-functional TDRS-A as a spare and the partially-functional TDRS-C designated to support NASA’s Compton Gamma Ray Observatory.

A few months before the launch of the final first-generation TDRS, in February 1995, NASA’s Goddard Space Flight Center of Greenbelt, Md., chose Boeing to build three second-generation satellites under a contract valued at $481.6 million. Based upon the 601 “bus,” the new TDRSes were intended to augment the Ku-band and S-band capabilities of the first generation with the higher-bandwidth Ka-band. The ground stations at White Sands were modified to accept the new satellites. TDRS-H was launched atop an Atlas booster in June 2000, followed by TDRS-I in March 2002 and TDRS-J the following December. Although TDRS-H suffered problems with its multi-access antenna and TDRS-I lost pressure in one of its four fuel tanks shortly after launch, the second generation has supported International Space Station and other operational assets for more than a decade.

Graphic representation of the second-generation TDRS in orbit. Three of these satellites were launched, atop Atlas II boosters, between June 2000 and December 2002. Image Credit: NASA

On 4 April 2013, NASA will mark the 30th anniversary of the TDRS-A launch. It has been a long and rocky road for a network which was born with such promise, but very soon fell on hard times, and matured to shine throughout the heyday of the shuttle era and today’s International Space Station. It has provided the data-relay capability for the astonishing scientific returns of the Hubble Space Telescope … and even the doddery first satellite has contributed to more down-to-earth achievements. In 1998, NASA allowed scientists at the Amundsen-Scott base in Antarctica to employ TDRS-A as a relay for research data and it supported a medical emergency at McMurdo Station, allowing scientists to conduct a telemedicine conference with doctors in the U.S. Several of the first-generation satellites are now out of service. TDRS-A—“the queen of the fleet,” according to NASA-Goddard’s Space Network Project Manager Roger Flaherty—was deactivated in October 2009, followed by TDRS-D in November 2011. The remaining first-generation satellites (C, E, F, and G) are expected to be retired by 2015. And by the end of that year, it is expected that the entire third-generation will be complete, with TDRS-K, L, and M inserted into geosynchronous orbit. These satellites are immeasurably more powerful than their predecessors, but, like them, they will enable TDRS to evolve through the middle of the next decade as the United States’ primary tracking and data-relay service provider for its key human-exploration programs and scientific endeavors.